1. Field of the Invention
The present invention relates generally to telemetry apparatuses and methods, and more particularly to robust networks for downhole telemetry repeater systems and methods for well drilling, production and completion, e.g., in the oil and gas and in other industries.
2. Description of the Related Art
Acoustic telemetry is a method of communication used in well drilling, production and completion. Applications include, but are not limited to, the oil and gas industry. In a typical drilling environment, acoustic extensional carrier waves from an acoustic telemetry device are modulated in order to carry information via the drillpipe as the transmission medium to the surface. Upon arrival at the surface, the waves are detected, decoded and displayed in order that drillers, geologists and others helping steer or control the well are provided with drilling and formation data. In production wells, downhole information can similarly be transmitted via the well casings or production string. Acoustic telemetry transmits data to surface in real-time and is independent of fluid flow, depth, well trajectory and other drilling parameters.
The theory of acoustic telemetry as applied to communication along drillstrings has generally been confirmed by empirical data in the form of accurate measurements. It is now generally recognized that the nearly regular periodic structure of drillpipe imposes a passband/stopband structure on the frequency response, similar to that of a comb filter. Dispersion, phase non-linearity and frequency-dependent attenuation make drillpipe a challenging medium for telemetry, the situation being made even more challenging by the significant surface and downhole noise generally experienced.
Drillstring acoustic telemetry systems are commonly designed with multiple transceiver nodes located at spaced intervals along the drillstring. The nodes can include repeaters. Acoustic telemetry networks can function in synch with the operation of the nodes and repeater nodes and other system components. Data packets consisting of drilling operation data are relayed node to node in a daisy-chain/linear fashion, typically beginning from a node located in the borehole apparatus (BHA), throughout the network to a destination, usually the surface receiver system. For purposes of minimizing interference between nodes, the data packets are transmitted (typically up-string) using time division multiplexing (TDM) techniques. Maximizing data packet transmission speed and throughput are objectives of drillstring telemetry systems and methods. For a discussion of a repeater network for these applications, see U.S. Patent Application Ser. No. 61/731,898, which is incorporated herein by reference.
When exploring for oil or gas, and in other well drilling, well completion and energy resource production operations, an acoustic transmitter is preferentially placed near the BHA, typically near the drill bit where the transmitter can gather certain drilling and geological formation data, process this data, and then convert the data into a signal to be transmitted, e.g., up-hole or in another direction, to an appropriate receiving and decoding station. In some systems, the transmitter is designed to produce elastic extensional stress waves that propagate through the drillstring to the surface, where the waves are detected by sensors, such as accelerometers, pressure transducers, etc., attached to the drillstring or associated drilling rig equipment. These waves carry information of value to the drillers and others who are responsible for steering the well. Examples of such systems and their components are shown in: Drumheller U.S. Pat. No. 5,128,901 for Acoustic Data Transmission through a Drillstring; Drumheller U.S. Pat. No. 6,791,470 for Reducing Injection Loss in Drill Strings; Camwell et al. U.S. Pat. No. 7,928,861 for Telemetry Wave Detection Apparatus and Method; and Camwell et al. U.S. Pat. No. 8,115,651 for Drill String Telemetry Methods and Apparatus. These patents are incorporated herein by reference.
The link performance within an acoustic repeater network is determined by the signal-to-noise ratio (SNR) and level of channel distortion. The links within a drillstring acoustic network are subject to large variations in channel distortion and noise that cause the occasional loss of signal packets. This loss represents a degradation in the network performance as quantified in average data throughput. The large noise variance means that the network performance remains degraded from full potential, even when average link quality is high. As the number of the nodes increases, the probability of lost packets increases exponentially according to the generalized formula:PSuccess_Network=(PSuccess_Link)#Links where PSuccess_Network is the probability of delivering a packet successfully throughout the network, Psuccess_Link is the probability of delivering a packet successfully across a single link in the network, and #Links is the number of inter-node communication links within the network.
Increasing the link SNR, either through increased transmitter power or reduced node spacing, can improve network performance. However, this is costly and lost packets, while reduced, will continue to persist on account of the large variation in noise and distortion.